The present invention relates to a manual appliance for cutting plastic pipes cylindrical or otherwise, of the type comprising a guide saddle closed by means of a completely opening lower saddle making it possible to take action on conduits in place as easily as on those being installed. Although there is no great difficulty in cutting plastic pipes in most common use at the present time, the same is not true of high-density polyethylene pipes which are supplied in the form of coils and which tend to be in more widespread use under the impetus of the Public Services in the following sectors: the transport of corrosive products, water supply, sewage disposal, the casing of defective conduits and gas distribution. These pipes are always ovalized to a great extent because of their method of delivery in the form of rings on reels, and as a result of their great thickness they have high rigidity. It is therefore virtually impossible to cut them on a wheel because with a wheel-type pipe cutter, whether it has a saddle or not, it is possible to cut only very slightly ovalized rigid pipes (such as steel pipes) or flexible pipes (such as those made of PVC) ovalized or otherwise and being subjected to deformation under the pressure of the wheel. A first solution has been proposed to solve this problem: this involves the use of shear-type bladed pipe cutters, but these, which are only heavy appliances resting on a base, still do not make it possible to work on the spot.